I don't have any details. I just heard they were planning a major rewrite when I reported a bug a long time ago (over 10 years ago) and in the bug tracker the conclusion was that fixing it should be done as part of the major rewrite that was upcoming.
For anyone curious, here's the bug (from memory).
I had a host with a POP3 mail server running on the normal POP3 port. That host also had a second POP3 mail server on a different port [1].
I tried to set up two email accounts in Thunderbird, one for a POP3 account for user tzs on that host on the standard POP3 port, and the other for tzs on that host on the second port.
The second account could not be created because it said that I already had a tzs POP3 account on that host. Thunderbird was using user/host as a unique account key, rather than the more correct user/host/port (or the even more correct user/host/protocol/port).
I reported this as a bug, figuring that this would be an easy fix. At worst somewhat tedious but no challenge. Soon someone marked the bug as a possible duplicate, linking to a bug from a couple years earlier that was also due to user/host being assumed unique.
In the discussion one of the developers said that the user/host as key assumption was hardwired into the code in too many places, but that was one of the things being redone in the upcoming rewrite.
[1] The second server wasn't actually on that host. That host was using an SSH tunnel to make my work POP3 server appear on my LAN.